/** \file mikktspace/mikktspace.h
 *  \ingroup mikktspace
 */
 /**
  *  Copyright (C) 2011 by Morten S. Mikkelsen
  *
  *  This software is provided 'as-is', without any express or implied
  *  warranty.  In no event will the authors be held liable for any damages
  *  arising from the use of this software.
  *
  *  Permission is granted to anyone to use this software for any purpose,
  *  including commercial applications, and to alter it and redistribute it
  *  freely, subject to the following restrictions:
  *
  *  1. The origin of this software must not be misrepresented; you must not
  *     claim that you wrote the original software. If you use this software
  *     in a product, an acknowledgment in the product documentation would be
  *     appreciated but is not required.
  *  2. Altered source versions must be plainly marked as such, and must not be
  *     misrepresented as being the original software.
  *  3. This notice may not be removed or altered from any source distribution.
  */

#ifndef __MIKKTSPACE_H__
#define __MIKKTSPACE_H__

#ifdef __cplusplus
extern "C" {
#endif

	/* Author: Morten S. Mikkelsen
	 * Version: 1.0
	 *
	 * The files mikktspace.h and mikktspace.c are designed to be
	 * stand-alone files and it is important that they are kept this way.
	 * Not having dependencies on structures/classes/libraries specific
	 * to the program, in which they are used, allows them to be copied
	 * and used as is into any tool, program or plugin.
	 * The code is designed to consistently generate the same
	 * tangent spaces, for a given mesh, in any tool in which it is used.
	 * This is done by performing an internal welding step and subsequently an order-independent evaluation
	 * of tangent space for meshes consisting of triangles and quads.
	 * This means faces can be received in any order and the same is true for
	 * the order of vertices of each face. The generated result will not be affected
	 * by such reordering. Additionally, whether degenerate (vertices or texture coordinates)
	 * primitives are present or not will not affect the generated results either.
	 * Once tangent space calculation is done the vertices of degenerate primitives will simply
	 * inherit tangent space from neighboring non degenerate primitives.
	 * The analysis behind this implementation can be found in my master's thesis
	 * which is available for download --> http://image.diku.dk/projects/media/morten.mikkelsen.08.pdf
	 * Note that though the tangent spaces at the vertices are generated in an order-independent way,
	 * by this implementation, the interpolated tangent space is still affected by which diagonal is
	 * chosen to split each quad. A sensible solution is to have your tools pipeline always
	 * split quads by the shortest diagonal. This choice is order-independent and works with mirroring.
	 * If these have the same length then compare the diagonals defined by the texture coordinates.
	 * XNormal which is a tool for baking normal maps allows you to write your own tangent space plugin
	 * and also quad triangulator plugin.
	 */

	typedef int tbool;
	typedef struct SMikkTSpaceContext SMikkTSpaceContext;

	typedef struct {
		// Returns the number of faces (triangles/quads) on the mesh to be processed.
		int (*m_getNumFaces)(const SMikkTSpaceContext* pContext);

		// Returns the number of vertices on face number iFace
		// iFace is a number in the range {0, 1, ..., getNumFaces()-1}
		int (*m_getNumVerticesOfFace)(const SMikkTSpaceContext* pContext, const int iFace);

		// returns the position/normal/texcoord of the referenced face of vertex number iVert.
		// iVert is in the range {0,1,2} for triangles and {0,1,2,3} for quads.
		void (*m_getPosition)(const SMikkTSpaceContext* pContext, float fvPosOut[], const int iFace, const int iVert);
		void (*m_getNormal)(const SMikkTSpaceContext* pContext, float fvNormOut[], const int iFace, const int iVert);
		void (*m_getTexCoord)(const SMikkTSpaceContext* pContext, float fvTexcOut[], const int iFace, const int iVert);

		// either (or both) of the two setTSpace callbacks can be set.
		// The call-back m_setTSpaceBasic() is sufficient for basic normal mapping.

		// This function is used to return the tangent and fSign to the application.
		// fvTangent is a unit length vector.
		// For normal maps it is sufficient to use the following simplified version of the bitangent which is generated at pixel/vertex level.
		// bitangent = fSign * cross(vN, tangent);
		// Note that the results are returned unindexed. It is possible to generate a new index list
		// But averaging/overwriting tangent spaces by using an already existing index list WILL produce INCRORRECT results.
		// DO NOT! use an already existing index list.
		void (*m_setTSpaceBasic)(const SMikkTSpaceContext* pContext, const float fvTangent[], const float fSign, const int iFace, const int iVert);

		// This function is used to return tangent space results to the application.
		// fvTangent and fvBiTangent are unit length vectors and fMagS and fMagT are their
		// true magnitudes which can be used for relief mapping effects.
		// fvBiTangent is the "real" bitangent and thus may not be perpendicular to fvTangent.
		// However, both are perpendicular to the vertex normal.
		// For normal maps it is sufficient to use the following simplified version of the bitangent which is generated at pixel/vertex level.
		// fSign = bIsOrientationPreserving ? 1.0f : (-1.0f);
		// bitangent = fSign * cross(vN, tangent);
		// Note that the results are returned unindexed. It is possible to generate a new index list
		// But averaging/overwriting tangent spaces by using an already existing index list WILL produce INCRORRECT results.
		// DO NOT! use an already existing index list.
		void (*m_setTSpace)(const SMikkTSpaceContext* pContext, const float fvTangent[], const float fvBiTangent[], const float fMagS, const float fMagT,
			const tbool bIsOrientationPreserving, const int iFace, const int iVert);
	} SMikkTSpaceInterface;

	struct SMikkTSpaceContext
	{
		SMikkTSpaceInterface* m_pInterface;	// initialized with callback functions
		void* m_pUserData;						// pointer to client side mesh data etc. (passed as the first parameter with every interface call)
	};

	// these are both thread safe!
	tbool genTangSpaceDefault(const SMikkTSpaceContext* pContext);	// Default (recommended) fAngularThreshold is 180 degrees (which means threshold disabled)
	tbool genTangSpace(const SMikkTSpaceContext* pContext, const float fAngularThreshold);

	// To avoid visual errors (distortions/unwanted hard edges in lighting), when using sampled normal maps, the
	// normal map sampler must use the exact inverse of the pixel shader transformation.
	// The most efficient transformation we can possibly do in the pixel shader is
	// achieved by using, directly, the "unnormalized" interpolated tangent, bitangent and vertex normal: vT, vB and vN.
	// pixel shader (fast transform out)
	// vNout = normalize( vNt.x * vT + vNt.y * vB + vNt.z * vN );
	// where vNt is the tangent space normal. The normal map sampler must likewise use the
	// interpolated and "unnormalized" tangent, bitangent and vertex normal to be compliant with the pixel shader.
	// sampler does (exact inverse of pixel shader):
	// float3 row0 = cross(vB, vN);
	// float3 row1 = cross(vN, vT);
	// float3 row2 = cross(vT, vB);
	// float fSign = dot(vT, row0)<0 ? -1 : 1;
	// vNt = normalize( fSign * float3(dot(vNout,row0), dot(vNout,row1), dot(vNout,row2)) );
	// where vNout is the sampled normal in some chosen 3D space.
	//
	// Should you choose to reconstruct the bitangent in the pixel shader instead
	// of the vertex shader, as explained earlier, then be sure to do this in the normal map sampler also.
	// Finally, beware of quad triangulations. If the normal map sampler doesn't use the same triangulation of
	// quads as your renderer then problems will occur since the interpolated tangent spaces will differ
	// eventhough the vertex level tangent spaces match. This can be solved either by triangulating before
	// sampling/exporting or by using the order-independent choice of diagonal for splitting quads suggested earlier.
	// However, this must be used both by the sampler and your tools/rendering pipeline.

#ifdef __cplusplus
}
#endif

#endif
